1. Field of the Invention
The present invention relates generally to an antenna provided in a mobile communication terminal to transmit and receive radio signals and, more particularly, to an ultra wideband internal antenna, which is provided within a mobile communication terminal and is capable of cutting off frequencies in a specific frequency band while processing ultra wideband signals.
2. Description of the Related Art
Currently, mobile communication terminals are required to provide various services as well as be miniaturized and lightweight. To meet such requirements, internal circuits and components used in the mobile communication terminals trend not only toward multi-functionality but also toward miniaturization. Such a trend is also applied to an antenna, which is one of the main components of a mobile communication terminal.
For antennas generally used for mobile communication terminals, there are helical antennas and Planar Inverted F Antennas (hereinafter referred to as “PIFA”). Such a helical antenna is an external antenna fixed on the top of a terminal and has a function of a monopole antenna. The helical antenna having the function of a monopole antenna is implemented in such a way that, if an antenna is extended from the main body of a terminal, the antenna is used as a monopole antenna, while if the antenna is retracted, the antenna is used as a λ/4 helical antenna.
Such an antenna is advantageous in that it can obtain a high gain, but disadvantageous in that Specific Absorption Rate (SAR) characteristics, which are the measures of an electromagnetic wave's harm to the human body, are worsened due to the omni-directionality thereof. Further, since the helical antenna is designed to protrude outward from a terminal, it is difficult to design the external shape of the helical antenna to provide an attractive and portable terminal. Since the monopole antenna requires a separate space sufficient for the length thereof in a terminal, there is a disadvantage in that product design toward the miniaturization of terminals is hindered.
In the meantime, in order to overcome the disadvantage, a Planar Inverted. F Antenna (PIFA) having a low profile structure has been proposed. FIG. 1 is a view showing the construction of a general PIFA.
The PIFA is an antenna that can be mounted in a mobile terminal. As shown in FIG. 1, the PIFA basically includes a planar radiation part 1, a short pin 3 connected to the planar radiation part 1, a coaxial line 5 and a ground plate 7. The radiation part 1 is fed with power through the coaxial line 5, and forms impedance matching by short-circuiting the ground plate 7 using the short pin 3. The PIFA must be designed in consideration of the length L of the radiation part 1 and the height H of the antenna according to the width WP of the short pin 3 and the width W of the radiation part 1.
Such a PIFA has directivity that not only improves Specific Absorption Rate (SAR) characteristics by attenuating a beam (directed to a human body) in such a way that one of all the beams (generated by current induced to the radiation part 1), which is directed to the ground, is induced again, but also enhances a beam induced in the direction of the radiation part 1. Furthermore, the PIFA acts as a rectangular microstrip antenna, with the length of the rectangular, planar radiation part 1 being reduced by half, thus implementing a low-profile structure. Furthermore, the PIFA is an internal antenna that is mounted in a terminal, so that the appearance of the terminal can be designed beautifully and the terminal has a characteristic of being invulnerable to external impact.
Generally, Ultra WideBand (UWB) denotes an advanced technology of realizing together the transmission of high capacity data and low power consumption using a considerably wide frequency range of 3.1 to 10.6 GHz. In Institute of Electrical and Electronic Engineers (IEEE) 802.15.3a, the standardization of UWB has progressed. In such a wideband technology, the development of low power consumption and low cost semiconductor devices, the standardization of Media Access Control (MAC) specifications, the development of actual application layers, and the establishment of evaluation methods in high frequency wideband wireless communication have become major issues. Of these issues, in order to execute a wideband technology in mobile communication applications, the development of a small-sized antenna that can be mounted in a portable mobile communication terminal is an important subject. Such an ultra wideband antenna is adapted to convert an electrical pulse signal into a radio wave pulse signal and vice versa. In particular, when an ultra wideband antenna is mounted in a mobile communication terminal, it is especially important to transmit and receive a radio wave without the distortion of a pulse signal in all directions. If the radiation characteristic of an antenna varies according to direction, a problem occurs such that speech quality varies according to the direction the terminal faces. Further, since a pulse signal uses an ultra wide frequency band, it is necessary to maintain the above-described isotropic radiation pattern uniform with respect to all frequency bands used for communication.
FIG. 2 is a view showing the construction of a conventional wideband antenna.
The antenna shown in FIG. 2 is a wideband antenna disclosed in U.S. Pat. No. 5,828,340 entitled “Wideband sub-wavelength antenna”. A wideband antenna 2 of the U.S. Patent includes a tap 10 having a tapered region 20, a ground plane 14 and a feeding transmission line 12 on a substrate 4. The bottom end 18 of the tap 10 has a width equal to that of a center conductor 12a of the feeding transmission line 12. The tapered region 20 is located between the top edge 16 and the bottom end 18 of the tap 10. Such a conventional wideband antenna has a frequency bandwidth of about 40%. However, when a radiation pattern in a horizontal plane, that is, a radiation pattern formed in y-z directions, is observed using a frequency function, the conventional wideband antenna exhibits isotropy in a low frequency band, but much radiation occurs in the transverse direction of the tap 10 (that is, a y direction) as the frequency increases. That is, the wideband antenna 2 is advantageous in that in an inexpensive planar wideband antenna can be implemented using Printed Circuit Board (PCB) technology, but problematic in that, as the frequency increases, serious distortion occurs and the antenna 2 has directionality. Further, the antenna is also problematic in that, since the size of the tap 10 emitting radiation is somewhat large, the tap 10 must occupy a large space in a mobile terminal.
Further, the conventional ultra wideband antenna 2 is problematic in that, since it uses frequencies in a 3.1 to 10.6 GHz wide frequency band, the operational frequencies of the frequency band of the ultra wideband antenna 2 overlap with those of other existing communication systems, thus interfering with communication therebetween. For example, since a wireless LAN uses frequencies in a 5.15 to 5.35 GHz wideband (US standard), the frequencies of the wireless LAN may overlap with those of the wideband antenna using the frequencies in the 3.1 to 10.6 GHz frequency band, thus interfering with the communication between respective communication systems.